seafood

I am familiar with the general frame geometry principles when it comes to steering characteristics and just now read through Sheldon Brown's primer on sizing. But the question I was trying to figure out remains unanswered. Help me out: why is that for a given rider, mountain bikes tend to use a more downward sloped top tube with a longer seat post, while road bikes use a more horizontal top tube that requires a shorter seat post? It can't be rigidity, since mountain bikes see greater vertical frame loads when they land jumps. It can't be mass, since a more compact frame would benefit a road bike as well. There seem to be no significant differences in terms of aerodynamics.

HillRider

No, the reason MTB's tend to use sloping top tubes and smaller frames for a given rider height is the necessity to provide standover clearance under the rider in harder and more uneven terrain. You want more clearance if you have to put your feet down to get over uneven trails. The longer seatpost is required since your relationship to the pedals remains the same while the frame is smaller and the seattube shorter. Roadbikes are used in more predictable and even terrain so the ability to reach the ground with little to no warning isn't as necessary.

FBinNY 

+1, the larger stand over clearance reflects the needs of riders on uneven terrain ans opposed to paved roads. If you ever had to do a dab in a hollow while the wheels were on higher ground, you'd instantly become a believer in more stand over clearance.

joejeweler

Simple really,...you MUST protect the family jewels.

3alarmer 

+4............Safety of your nuts in rough terrain is directly proportional to distance from that top tube.

krome

Clearance for the huge balls one must have for some of the off road hijinks that occur.

fietsbob

The bike biz is also making some road frames with the sloping top tube,
and before the suspension forks became standard on MTB's,
they had horizontal top tubes, too..

A part of the bike 'geometry' is the relative length of seat tube, aka 'size' and the length of the top tube,
the measurement if the top tube slopes , starts up the seatpost, to a height as if the top tube, were horizontal

throw in the seat tube angle and somehow you end up with a balance of your weight between the wheels, ..

frantik

besides standover, it also benefits the manufacturers because they don't have to make as many sizes

HillRider

Yes and that's what driven the trend toward sloping top tubes on road frames. That, and the fact that a sloping toptube frame is generally slightly lighter than it's horizontal toptube equivalent and allows advertising it as such.

well biked

While all of this is true, do keep in mind that mountain bikes started out with horizontal top tubes, and remained that way for quite a few years.......Now, we have a shop full of new bikes, and not a single one of them that I can think of has a horizontal top tube. There are some in the industry, but not that many. Road, mountain, hybrid doesn't matter, almost all of them have some degree of sloping top tube these days.

Regarding mountain bikes, with the 29er becoming more and more common, what you'll see is some very unconventional top tube designs on a lot of bikes. The front end is very tall on these bikes, and so standover clearance is an issue. With aluminum frames especially, with hydroformed tubes, you'll see a lot of curvy top tubes that are strategically shaped to create some clearance where the rider stands over the bike.

HillRider

Correct but back then MTB's were typically sized significantly "smaller" that a road bike for the same rider. I had a '92 Trek 7000 rigid fork, hardtail MTB with a horizontal top tube and was sized for an 18" (46 cm) frame while I ride a 56 or 57 cm road frame. The small frame allowed adequate standover clearance while the top tube was proprotionally longer than the same size road frame to give a proper reach to the bars..

well biked

Yes, I was just pointing out that mtb frames haven't always used sloping top tubes, and you're right, a properly fitted mountain bike in those days would still account for providing adequate standover clearance. The key point being "properly fitted" of course.

Andrew R Stewart 

If you go back to the beginning of the safety bike you'll see top tubes sloped often enough. To slope or not is a fashon thing as much as a design/clearance thing. These days it's Ok to slope (if fact a horizontal top tube is considered "old school") but just 20-30 years ago a sloping TT was weird.

The 11th frame I built, in 1981, had a sloping TT due to sizing issues. The customer thought this was just grand. fellow riders took oit as strange. Andy.

bradtx

seafood, The UCI has rules for road bikes used for racing and those rules maybe the one item keeping a road frame from looking more like a mountain bike frame. Maybe keeping a mountain bike frame from looking like a BMX frame also?

Anyway the long sloping top tube on a mountain bike primarily allows generous body english and toe dabbing ability which generally isn't often applied on a road bike.

Brad

seafood

Great info so far, thanks everyone! I come from the motorbike world and I see some interesting parallels with the design direction dichotomy between dirtbikes and sportbikes as far as suspension travel, ground clearance, trail, and body position and control ergonomics. Bicycles generally have more freedom in frame design because the frame doesn't need to carry anything in it, except to locate the critical axes and the lower forces being put into them allows for more choices in materials and construction.

The standover requirements for mountain bikes seem peculiar to me when I compare them to dirtbikes. Being primarily a road rider -- with or without a motor -- I admit I may be totally out of the loop, but it appears that the need to reach uneven ground, support a big-travel fork, and protect family joules is roughly the same in both disciplines. But I can see the mass centralization and savings that such a design gives mountain bikes.

The thing about road bikes being out of lock-step with their frame design due to fashion or seemingly arbitrary competition rules is a bit disheartening. But oh well.

Andrew R Stewart 

Agree that the focus on stand over clearance on a dirt bicycle seems to me to be a bit too much. When ever I dab the other foot is on it's pedal. Andy.

fietsbob

There is the dab on the down hillside of an off camber slope , that will have you considering
the contact with the mini-me..
stick to fire and logging, roads rather than un-groomed, un-popular single track.
and you should be fine..

FBinNY 

It really depends on how technical the course. Mess up hoping a 12" log and have to stop with the front wheel up there, and you could appreciate the added clearance.

Also consider that bikes aren't designed for top skilled riders. They have to be designed for the general public, and so there has to be as much margin for error as possible, since we know up front that there will be errors.

oldbobcat

The smaller size was partly to make up for the higher bottom brackets on MTBs. And riser stems brought the handlebar up around saddle level or higher. Nowadays, with 100-160 mm suspension systems and even higher bottom brackets, head tubes are higher (but not longer), so more creativity is needed to address standover height.

Regarding road bikes, experimental designs have emerged from the '70s through the '90s, and here's what we know. The traditional riders' position that we've used for the last 80 years is pretty optimum for long days in the saddle over varied terrain. Tweaks have appeared and some of them have endured, but rules notwithstanding, there's little reason for change. Looking more closely at the geometry of the frame, there's little reason to abandon the basic double-triangle design. There's plenty of room for experimentation with compact, semi-compact, traditional, curved top tubes, curved down tubes, and curved or squiggly forks and stays. Basically, those Y-frames from the early '90s sucked.

Andrew R Stewart 

For me, when climbing over logs i found the stem to be the larger impact zone. Maybe i have a more forward projection and less downwords one. Andy.

FBinNY 

Possibly. It's just that I've watched too many people make it half way, then stall or chicken out then fall because they couldn't touch the ground (even with low mtb ground clearance).

Jeff Wills

Bicycles in general have changed very little in gross terms in the last 40 years. Eddy Mercyx on his Molteni would not look out of place in today's peloton.

I had a parallel thought when I read reactions to early reviews to the Honda NC700X: when the press praised its "radical new features", many people pointed out that they were nothing new in the motorcycle world.

"Family joules"... interesting pun, that.

FBinNY 

There's a reason for that.

Back some 30+ years ago, when the Americans initiated the practice of designing bikes with technical advantages like fairings, and unique frame geometry, it became a sort of arms race, and the UCI initiated rules freezing in place the basic, unfaired double triangle in use at the time. If it were not for that rule we'd see the diversity of road frames in Road competition that we see in Tri and mtb racing.

Carbon especially opens up opportunities to depart from the traditional design, and the only reason carbon frames are designed along the lines of metal tubing frames, is the UCI rules.

seafood

This strikes me as kinda sad. Not necessarily because I want tech for tech's sake, but why stifle innovation? I remember reading about Graeme Obree and I still can't get my head around what seems like nothing but good old boys running a good old boys' club. Straight out of Wikipedia: "The world governing body, the Union Cycliste Internationale grew concerned that changes to bicycles were making a disproportionate improvement to track records." And of course there's this doozy of a summation:
Am I the only one who thinks this is insane? Seems every other competitive discipline affected by technological innovation deals with this by sanctioning both loose rule books that encourage experimentation, as well as spec series and vintage competitions without causing riots in the streets.

3alarmer 

You're talking about bicycle racing......................think about it.